1. Field of the Invention
The invention relates to an apparatus and method for placing a device into a test mode. In particular, the invention relates to an apparatus and method for placing a quad subscriber line audio processing circuit (QSLAC) into a test mode by inverting the functions of two pins for that device.
2. Description of the Related Art
Various methods have been devised to place different kinds of devices into a test mode. Generally, these methods involve utilizing "test signal" inputs to the devices in order to invoke the test mode.
One type of device that is commonly placed into a test mode is a subscriber line audio processing circuit (SLAC), or a quad SLAC (QSLAC). A SLAC or QSLAC is used with telephones, and it handles analog-to-digital signal conversion, filtering, and interfacing with a pulse code modulation (PCM) highway. Such devices are described, for example, in "Integrated Ring Detection Circuit and Power Cross Detection Circuit With Persistence Timers", U.S. Pat. No. 5,636,273, invented by W. Schopfer, and assigned to AMD Inc., which is incorporated herein by reference.
In order to place a SLAC or QSLAC into a test mode, a test signal that is connected to a test input pad is set to an active state. However, such a method of setting the device into the test mode is prone to problems, in that an inadvertent setting of the test input pad into the active state would place the device into the test mode when the normal mode is desired.
Testing is an integral part of a design process for a VLSI circuit, such as a SLAC or a QSLAC. Testing should be accommodated with primary design functionalities in order to insure proper device operation. In order to achieve these testing goals, a device under test is often programmed to enter one or more modes in which the operation of one or more of the device's sections is evaluated. Once the device is in a particular test mode, a predefined sub-section is exercised and all important signals corresponding to this sub-section of the device are brought out to the external pins for observation. A test mode thus allows one to debug and verify a sub-block or one of the functionalities of the device. Normally, a device may enter several test modes. Different functionalities are individually addressed in each of these test modes. Depending upon a particular test mode selected, the functionality of the device may change. The pin-mapping of the device may also change, causing the function behavior and desired performance characteristics of various pins to change.
Accommodating all such test requirements without changing the normal operation of the device requires programming the device in the test mode. Entering the test mode requires communication with the device. If the device is an externally programmable part, test engineering can program the device to enter the test mode and conduct the required testing. Commands to execute testing can be provided through pins available for external programming. The end user, however, is often not provided this information so as not to enter the test mode.
When the part is not externally programmable, there is no predefined communication channel through which test engineering can communicate with the device. The functional description and the behavior of each pin is pre-characterized and cannot be altered to insert the test mode. In such a case, another option is to visit each pin on the device, and use a pin strapping technique on an available pin to control the operational mode of the device. In pin strapping, an external input pin, not used otherwise, is either tied to VCC or GND. Depending upon which voltage supply the pin is tied to, the device can be guided to operate either in a test mode or a normal operating mode. However, this method requires that a pin be available for strapping. Hence, the device is either programmable or has some external pin available for the test mode entry.
It is desirable to provide a scheme for placing such a device, and other types of devices, into a test mode without needing to add new test ports for the device.